C++ 11-作用域和全局变量

Question

如何从内部范围访问全局变量，给定以下代码示例，如何从主函数和最内部范围访问全局字符串 X，也是最内部范围可访问一旦我们将它退出到主范围或其他范围？

#include <iostream>
#include <string>
std::string x = "global";
int counter = 1;

int main()
{
    std::cout <<counter ++ << " " << x << std::endl;
    std::string x = "main scope";
    std::cout <<counter ++ << " "  << x << std::endl;
    {
        std::cout <<counter ++ << " " << x << std::endl;
        std::string x = "inner scope";
        std::cout <<counter ++ << " " << x << std::endl;
    }
    std::cout <<counter++ << " "  << x << std::endl;
}

当前 cout 是：

1 global
2 main scope
3 main scope
4 inner scope
5 main scope

Answer 1

可以使用 ::x 达到全局范围，根据：

#include <iostream>
#include <string>

std::string x = "global";
int counter = 1;

int main()
{
    std::cout << counter++ << " " << x << std::endl;
    std::string x = "main scope";
    std::cout << "  " << ::x << std::endl;
    std::cout << counter++ << " "  << x << std::endl;
    {
        std::cout << "  " << ::x << std::endl;
        std::cout << counter++ << " " << x << std::endl;
        std::string x = "inner scope";
        std::cout << "  " << ::x << std::endl;
        std::cout << counter++ << " " << x << std::endl;
    }
    std::cout << "  " << ::x << std::endl;
    std::cout << counter++ << " "  << x << std::endl;
}

这给你：

1 global
  global
2 main scope
  global
3 main scope
  global
4 inner scope
  global
5 main scope

hard 位实际上进入中间范围，例如 main scope 当您使用内部范围时。

一种方法是使用引用：

#include <iostream>
#include <string>

std::string x = "outer";

int main()
{
    std::cout << "1a " << x << "\n\n";

    std::string x = "middle";
    std::cout << "2a " << ::x << '\n';
    std::cout << "2b " << x << "\n\n";

    {
        std::string &midx = x;  // make ref to middle x.
        std::string x = "inner";  // hides middle x.
        std::cout << "3a " << ::x << '\n';
        std::cout << "3b " << midx << '\n';  // get middle x via ref.
        std::cout << "3c " << x << "\n\n";
    }
}

给出：

1a outer

2a outer
2b middle

3a outer
3b middle
3c inner

但是，作为一个好建议，如果您：

，您会发现附近不会有那么多问题：

更智能地命名您的变量，以便避免冲突；和
避免像瘟疫一样的全局变量:-)

并且，对于内部作用域中的变量，一旦您离开该作用域，它们将不再可用，即使有一个引用（您可以将它们复制到一个变量中更大的范围，但这与访问内部范围变量不同）。

C++ 11-作用域和全局变量

C++ 11- scopes & global variables

scopes

c++11